I once had a similar problem on another vehicle and it ended up being a bad alternator.

It was conducting power to ground through a fried diode network. I found it buy accident when I went to check on the car after an all night charge on the battery.

Before starting it up the alternator was found to be nearly red hot from all the power is was consuming all night long with the charger there to keep up the battery. The rest of the motor was stone cold of course. The alternator was pulling about 10 amps anytime it was connected. This was not enough to blow the fusible link in the system but plenty enough to kill the battery overnight.

Try to remove the #1 fusible link and see if the "drain" is gone. That fusible link is the lowest one in the fusebox left corner as you stand in front of the car, usually yellow, and it's BOLTED in place. You can't just wiggle it out since it's a 120A fuse.

It might actually be easier to lay under the car and disconnect the two big white wires at the alternator instead. While you are under there, see if the alternator housing is hot when it should be cold.